1. Field of the Invention
The present invention relates to a quantitative method for determining trace amounts of silicone oil and siloxane in fumed silica or pyrogenic oxides. The present invention involves both a procedure for preparing the sample for analysis and a method of analyzing the prepared sample by proton nuclear magnetic resonance spectroscopy. The present technique results in greatly improved sensitivity in the determination of the concentration of silicone oil and/or siloxane in fumed silica.
2. Background Information
It has been known that even trace amounts of silicone oil and/or siloxane in silica formulations for use in lacquer, paint or adhesive compositions, or other compositions and applications related to xe2x80x9cpaintabilityxe2x80x9d and/or adhesion, can cause serious adhesion problems for such coatings. These problems occur even when the silicone oil or siloxane impurities are present in very small or trace amounts, on the level of ppm (parts per million). These problems are due to the silicone oil or siloxanes acting as, for instance, phase separators in the paint or adhesive.
Silica formulations that are functionalized with siloxanes are generally not used in applications which require high levels of adhesiveness. However, in the mass production of post-treated silica it is almost inevitable that traces of siloxane will be present in silica formulations, for instance, resulting from contamination due to product changeover in a production line.
Until the present invention, no analytical technique was known that allowed for direct quantitative measurement of the siloxane traces in the silica. Conventional methods such as infrared spectroscopy are not sensitive enough. The technique of proton nuclear magnetic resonance spectroscopy (1H-NMR), which includes putting the silica in a deuterated solvent, fails to produce quantitatively measurable spectra because of signal broadening. Recently, attempts have been made with more expensive, very new methodsxe2x80x94such as TOF-SIMS (Time of Flight-Secondary Ion Mass Spectrometry) to indicate the presence of siloxane traces on a cleaned silica surface. This indication was only successful on a damaged paint layer. Apparently the concentration of siloxane in the silica gel is too low to be measured by this technique.
A method for the quantitative determination of silicone oil or siloxanes in trace quantities in silicas or pyrogenic oxides by the quantitative evaluation of 1H-NMR spectra by dispersing the silica or pyrogenic oxide in an organic solvent, then after isolating the solids, evaporating the clear phase to dryness and after taking up the residue in a deuterated solvent, quantitatively evaluating the silicone oil or siloxane via the integral of the methyl silyl protons indicated on the 1H-NMR spectrum.
The present invention, however, allows for the first time quantitative determination of silicone oil or siloxane traces in silica with high sensitivity. This method can be used in special situations and in a production environment can be a criterion for barring release or releasing materials for production use. Use of the present method can help to mitigate or avoid paint damages, of a significant size (sometimes well above 1 million DM) beforehand.
In order to obtain quantitative measurements in the desired sensitivity range, it is necessary to follow the steps of the sample preparation method outlined herein. The separation of the solid silica from the dissolved silica isolates the silicone oil and siloxane species from the silica. The subsequent evaporation and redissolution steps result in a solution with a higher concentration of the species of interest.
Since silica can adsorb certain amounts of siloxane reversibly, the total siloxane concentration is determined with the aid of a calibration line, which is determined with the use of silica samples that have a known concentration of siloxane.